Polycarbonates have been used in various applications because of excellent heat resistance, mechanical properties, dimensional stability and transparency thereof. Ordinary conventional polycarbonates have a linear molecular structure. The polycarbonates having such a molecular structure tend to show a large drawdown, etc., namely be deficient in melt tension upon extrusion molding, especially when extrusion-molded using a sizing die having a function of cooling a molten resin extruded from an extruder while forming the molten resin into a desired shape. When a molecular weight of the polycarbonates is increased to improve melt properties thereof such as drawdown, a melt viscosity of the polycarbonates becomes too high upon extrusion molding, so that a load applied to the extruder is too large, resulting in defects such as poor productivity. Therefore, there has been a demand for improving these properties of the polycarbonates.
As the method of improving the melt properties of the polycarbonates such as drawdown, there is known the method of branching polycarbonates by interfacial polymerization method using 2,2-bis(4-hydroxydiphenyl)propane (hereinafter referred to merely as “bisphenol A” or “BPA”) together with a polyfunctional compound as a branching agent such as 1,1,1-tris(4-hydroxyphenyl)ethane (THPE) and 1,3,5-tris(4-hydroxyphenyl)benzene (e.g., Japanese Patent Publication (KOKOKU) Nos. 44-17149 and 47-2918, Japanese Patent Application Laid-Open (KOKAI) Nos. 2-55725 and 4-89824, etc.). However, in order to obtain branched polycarbonates having melt properties sufficient to stably produce profile extrusion-molded products having a complicated shape using a die (for example, twin-wall molded products), it is required to copolymerize a large amount of the polyfunctional compound with the polycarbonates, resulting in problems such as discoloration of the resultant molded products, contamination of production lines and increased costs owing to the polyfunctional compound used in a large amount.
To solve these problems, various attempts have been conducted in transesterification methods using a carbonic diester and an aromatic dihydroxy compound (for example, Japanese Patent Application Laid-Open (KOKAI) Nos. 4-89824 and 6-136112, Japanese Patent Publication (KOKOKU) Nos. 7-37517 and 7-116285, etc.). However, these methods tend to be still unsatisfactory due to decomposition of the branching agent at a high temperature which results in failure to exhibit a sufficient branching effect, as well as occurrence of discoloration upon melt molding.